Ecological effects of reduced versus oxidised nitrogen deposition

世纪后半叶，欧洲大气中氮化合物的沉积急剧增加。在同一时期，各种植被类型的物种组成发生了重大变化，许多特征物种消失，这不能用管理变化等因素来解释。从20世纪90年代初起，有证据表明，氮沉降的增加与物种组成的一些变化有关。现在人们普遍认识到，过去50年来大气氮沉降的增加是各种生态系统中许多特征植物物种急剧减少的主要因素。氮沉降可以通过增加土壤的酸度来起作用，因为碱性阳离子如钙被释放并从系统中浸出;这可能导致植物中的碱性阳离子缺乏。它还增加了土壤中可用氮的数量，这一过程通常被称为富营养化。已知盐基阳离子缺乏和富营养化都是特征物种衰退的机制。氮沉降主要由两种形式组成：还原态氮，主要来自动物粪便和粪肥;氧化态氮，主要来自燃料燃烧。重要的是要了解这些形式的氮具有最大的生态影响，使政策，以控制排放和保护敏感物种是有效的，特别是因为有很大的空间差异，在整个欧洲的减少和氧化的氮沉积率。到目前为止，很少有实验或实地研究适当地比较了还原和氧化氮沉降的不同影响，甚至更少的考虑了与土壤酸度相互作用的重要性。然而，最近的研究表明，减少，但不氧化，氮的沉积量增加是有害的许多植物物种从敏感的生态系统，特别是当这些生态系统已经成为酸化。本研究的目的是：（i）确定还原态氮（而非氧化态氮）在敏感生态系统中的大气沉积差异是否解释了近几十年来北方欧洲植物物种组成的变化;（ii）确定导致氮沉积减少和/或还原态氮/氧化态氮高比值的机制，当土壤中的盐基阳离子水平较低时，这种机制具有更大的影响;和（iii）利用这些信息来确定生境，物种和位置的最大风险，从氮沉降。我们将使用实验室和现场实验相结合，并从实地调查在英国和荷兰的数据分析，以满足这些目标。

Summary Atmospheric deposition of nitrogen compounds has increased dramatically in Europe in the 2nd half of the 20th century. Over the same period, important changes in species composition of various vegetation types, and the loss of many characteristic species, have occurred which cannot be explained by factors such as changes in management. From the early 1990s onwards, evidence was found that increased nitrogen deposition was associated with some of these changes in species composition. It is now widely recognised that the increased atmospheric nitrogen deposition over the past 50 years is a major factor in the sharp decline of many characteristic plant species from a variety of ecosystems. Nitrogen deposition can act by increasing the acidity of soil, because base cations such as calcium are released and leach out of the system; this can lead to base cation deficiencies in plants. It also increases the amount of available nitrogen in the soil, a process which is often referred to as eutrophication. Both base cation deficiency and eutrophication are known to be mechanisms involved in the decline of characteristic species. Nitrogen (N) deposition is largely composed of two forms: reduced N, which is emitted mainly from animal waste and manure, and oxidised N which is emitted mainly from fuel combustion. It is important to understand which of these forms of nitrogen has the greatest ecological impacts, so that policies to control emissions and protect sensitive species are effective, especially as there are large spatial differences in the rates of reduced and oxidised N deposition across Europe. Very few experimental or field studies up to now have properly compared the different effects of reduced and oxidised N deposition, and even fewer have considered the importance of interactions with soil acidity. However, recent research has shown that elevated deposition of reduced, but not oxidised, N is detrimental for many plant species from sensitive ecosystems, especially when these ecosystems have already become acidified. The aims of this study are:- (i) to determine whether differences in the atmospheric deposition of reduced, rather than oxidised, nitrogen to sensitive ecosystems accounts for the changes in plant species composition seen in northern Europe in recent decades; (ii) to identify the mechanisms which lead to reduced nitrogen deposition, and/or high reduced/oxidised N ratios, having a greater impact when soils have low levels of base cations; and (iii) to use this information to identify habitats, species and locations at greatest risk from N deposition. We will use a combination of laboratory and field experiments, and data analysis from field surveys in both the UK and The Netherlands, to meet these aims.